The present invention generally relates to a null symbol position detecting method and a null symbol position detecting apparatus for detecting a null symbol from signals such as DAB (Digital Audio Broadcast) signal for example and to a receiver for receiving such a signal including null symbols.
Europe has started the provision of digital audio broadcast services of the European standard (Eureka 147) called DAB. DAB transmits a broadcast signal (or an ensemble signal) formed by multiplexing audio data of plural broadcast programs, various control information, and additional information. In this case, the audio data are encoded highly efficiently (data-compressed) by an international standard MPEG audio scheme to be transmission-line encoded by use of error-correction convolutional coding for example.
The bit streams of the encoded audio data and information are interleaved and modulated by OFDM (Orthogonal Frequency Division Multiplex) before being transmitted.
This DAB broadcast signal has a frame structure as shown in FIG. 5. Namely, each frame consists of a synchronization channel, an FIC (Fast Information Channel), and an MSC (Main Service Channel).
As shown in FIG. 5, there are two synchronization symbols. A null symbol, one of the synchronization symbols, indicates the start position of each frame. In this null symbol, a TII (Transmitter Identification Information) is inserted by use of plural carriers. However, the null symbol has no information to be transmitted as data. A synchronization symbol that follows the null symbol provides synchronization information for obtaining frequency offset and time offset in the receiver.
There are three symbols in FIC (block 1 through block 3) for transmitting various pieces of broadcast-associated information if a DAB signal of mode 1 to be described is taken as example. For example, these FIC blocks convey an array of multiplexed broadcast programs, labels such as broadcast station name (ensemble label) and program name (program label), and information indicative of broadcast program type.
The MSC has data fields (transmission symbols) of plural multiplexed broadcast programs as shown in FIG. 5. Namely, plural pieces of audio data multiplexed on a DAB signal are the MSC data fields. Each of the MSC data fields consists of a guard band (or a guard interval) and a valid symbol as shown in FIG. 5.
The information of each guard band period is made equal to a part of the period of each valid symbol, for example, the information of a period indicated by period DK at the end of each valid symbol as shown in FIG. 5. This guard band significantly improves the quality of reception of a DAB signal on a moving vehicle like an automobile in such highly radio-reflective environments as urban and hilly districts, as compared with other modulation schemes. Namely, as long as a time difference between the first and last reflected waves does not exceed the guard band period, no inter-symbol interference takes place and there is no need for waveform equalization.
A DAB receiver uses the above-mentioned FIC-included information for broadcast program identification and the information about the multiplexing to extract and reproduce a user-specified broadcast program from the DAB signal multiplexed with plural broadcast programs, allowing the user to listen to audio broadcast of high sound quality.
In order to receive the above-mentioned DAB signal to tune in to a desired broadcast program, the beginning of each DAB signal frame must be identified to accurately extract the information that follows the synchronization symbol. Therefore, null symbol detection is executed in the receiver.
Normally, a null symbol position is detected by analog signal processing. Namely, when a DAB signal is received and envelope-detected, the level of the envelope waveform becomes low in the null symbol portion. Therefore, by comparing the envelope waveform with a reference voltage, a null detection signal can be generated. FIG. 6A shows a waveform of a received DAB signal, in which the low-level portion indicates the null symbol portion.
As a result of the comparison between the DAB signal (the received signal) and the reference signal shown in FIG. 6A, a null detection signal is formed in which it goes low during the period of the null symbol and high in the other portions as shown in FIG. 6B.
However, if the reference signal is set improperly or if a DAB signal is received on a moving vehicle on which an unwanted phenomenon such as fading takes place, the received signal is disturbed, resulting in poor reception of the DAB signal or causing a null detection signal to occur at positions where it should not.
For example, when a moving vehicle such as an automobile is moving at a high speed and a DAB signal is received on this moving vehicle, the envelope of the received signal becomes broad and narrow as shown in FIG. 7A. Namely, the envelope of the received signal becomes narrow in other portions than the null symbol period.
If this happens, the null detection signal becomes low indicative of the detection of a null symbol also at positions other than where it should be present, thereby causing null symbol erroneous detection, thereby failing quick and correct null symbol detection.
It is therefore an object of the present invention to provide a null symbol position detecting method, a null symbol position detecting apparatus based on this method, and a receiver for receiving a broadcast signal including a null symbol, these being capable of quick and correct null symbol detection in any situation when using the broadcast signal including a null symbol.
In carrying out the invention and according to one aspect thereof, there is provided a null symbol position detecting method for receiving a signal and detecting a null symbol position from the received signal, in the signal of which a unit transmission frame has at least a null symbol at the beginning of the unit transmission frame and a plurality of transmission symbols following the null symbol, each of the plurality of transmission symbols having a guard band and a valid symbol, information included in a period of the guard band is made equal to information included in the valid symbol at an end thereof, and a length of a period of the null symbol is made different from a length of a period of each of the plurality of transmission symbols, the method comprising the steps of computing a correlation value between the received signal and a signal obtained by delaying the received signal by a length of a period of the valid symbol, and detecting the null symbol position on the basis of a variation in a period in which the correlation value is computed above a predetermined level.
According to the null symbol position detecting method, a signal multiplexed with a null symbol and one or more transmission symbols in a time division manner is received. A correlation value between the received signal and a signal obtained by delaying the received signal by one valid symbol period is computed. Because of the length of null symbol period is different from the length of transmission symbol period, the period in which a correlation value higher than a certain level is computed varies even within a unit transmission section (or a frame). A null symbol position is detected on the basis of this variation.
Thus, a null symbol position is detected on the basis of the variation in the period in which a correlation value higher than a certain level is computed. This permits the quick and accurate detection of null symbol position without being affected by the variation in the reception level of a received signal due to fading for example, always ensuring the quick and accurate null symbol position detection.
In carrying out the invention and according to another aspect thereof, there is provided a null symbol position detecting method, wherein a moving average for a period equal to the period of the guard band is computed for the correlation value between the received signal and a signal obtained by delaying the received signal by a length of a period of the valid symbol and the null symbol position is detected on the basis of a variation in a period in which the computed moving average peaks.
According to the null symbol position detecting method, a correlation value between the received signal and a signal obtained by delaying the received signal by one valid symbol period is computed. Because of the length of null symbol period is different from the length of transmission symbol period, the correlation value occurrence pattern varies within the unit transmission section in time or in correlation value level. The null symbol position is detected on the basis of this correlation value occurrence pattern.
Thus, a null symbol position is detected on the basis of correlation value occurrence pattern. This permits the quick and accurate detection of null symbol position without being affected by the variation in the reception level of a received signal due to fading for example, always ensuring the quick and accurate null symbol position detection.
In carrying out the invention and according to still another aspect thereof, there is provided a null symbol position detecting method, wherein the null symbol position is detected on the basis of a time interval in which the moving average peaks.
According to the null symbol position detecting method, the information of guard band period is made equal to the information of a portion (end portion) at the end of a valid symbol following that guard band, the portion (period) having the same length as the guard band period.
Therefore, for the correlation value between the received signal and a signal obtained by delaying the received signal by the valid symbol period, a moving average is computed for a period equal to the guard band period. Consequently, the value of the moving average of the correlation value is peaked at the end of each transmission symbol (namely, in the boundary between transmission symbols).
In the period in which transmission symbols continue, a peak-to-peak period of moving average correctly matches the transmission symbol period but, in the null symbol period, the peak level goes low. Therefore, the peak variation period and the peak occurrence pattern vary regularly, comparatively facilitating null symbol detection.
Thus, the null symbol position in the received signal can be quickly and accurately detected by use of the peak of the moving average of the correlation value as a correlation value and on the basis of the variation in the period of this peak or on the basis of the peak occurrence pattern.
In carrying out the invention and according to yet another aspect thereof, there is provided a null symbol position detecting method for receiving a signal and detecting a null symbol position from the received signal, in the signal of which a unit transmission frame has at least a null symbol at the beginning of the unit transmission frame and a plurality of transmission symbols following the null symbol, each of the plurality of transmission symbols having a guard band and a valid symbol, information included in a period of the guard band is made equal to information included in the valid symbol at an end thereof, and a length of a period of the null symbol is made different from a length of a period of each of the plurality of transmission symbols, the method comprising the steps of computing a correlation value between the received signal and a signal obtained by delaying the received signal by a length of a period of the valid symbol, computing, for the correlation value, a moving average for a period equal to the period of the guard band to detect peaking of the moving average, and detecting the null symbol position on the basis of an occurrence pattern in which the moving average peaks and a time interval in which the moving average peaks.
According to the null symbol position detecting method, the information of guard band period is made equal to the information of a portion (end portion) at the end of a valid symbol following that guard band, the portion (period) having the same length as the guard band period.
Therefore, for the correlation value between the received signal and a signal obtained by delaying the received signal by the valid symbol period, a moving average is computed for a period equal to the guard band period. Consequently, the value of the moving average of the correlation value is peaked at the end of each transmission symbol (namely, in the boundary between transmission symbols).
In the period in which transmission symbols continue, a peak-to-peak period of moving average correctly matches the transmission symbol period but, in the null symbol period, the peak level goes low. Hence, first, the reception of the null symbol is detected on the basis of the occurrence pattern of the moving average peak. Then, the peak-to-peak time interval is obtained. It is determined whether this time interval includes the null symbol period, thereby accurately detecting the null symbol position in the received signal.
The use of the moving average peak of the correlation value between received signals allows the detection of null symbol position without being affected by the reception level of the received signal. And the use of the peak-to-peak time interval allows the accurate detection of null symbol position without involving erroneous detection.